The aim of this trial is to evaluate the efficacy and safety of 2.5 and 5 mcg tiotropium compared to placebo over 12 week treatment period. Tiotropium inhalation solution will be delivered via Respimat inhaler and will be examined on top of maintenance inhaled corticosteroid treatment in patients with mild persistent asthma. Efficacy and safety will be assessed by measuring the effects on lung functions, effects on lung exacerbations, effects on asthma control and numbers of adverse events.